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Sunny D: Sunlight and Vitamin D

Touted by experts as the latest super-supplement, vitamin D may help ward off aches and pains, banish fatigue, increase muscle power, and slow weight gain, all while strengthening bones. It's an impressive list of...

Touted by experts as the latest super-supplement, vitamin D may help ward off aches and pains, banish fatigue, increase muscle power, and slow weight gain, all while strengthening bones. It’s an impressive list of pros--so why, then, did a recent study find that more than 40 percent of teens suffer from vitamin D deficiency?

The answer is simple: The body can’t make vitamin D on its own. Few foods naturally contain the nutrient (fish like salmon and mackerel and foods like liver and egg yolks do). For that reason, public health officials mandated decades ago that milk be fortified with vitamin D. But reliance on milk may have caused part of the problem, says Darshak Sanghavi, M.D., of the University of Massachusetts. "Today teens have more beverage options than before, like soda, energy drinks, and coffee. Those have replaced milk as the central drink in their diet," she explains, "so they don’t get the same amount of D that they used to. Plus, many people of color--including African-Americans, Asians, and Latinos--suffer from lactose intolerance and avoid milk altogether."

Certain diets may play a role in vitamin D deficiency as well. "Teen girls are more likely to experiment with diets like veganism, which cut out products such as eggs, fish, and fortified dairy, the common dietary sources of vitamin D," says Lisa Callahan, M.D., of the Women’s Sports Medicine Center at New York’s Hospital for Special Surgery. But food is just one way the body gets vitamin D.

The most abundant source of vitamin D by far is the sun (the body uses sunlight to convert a form of cholesterol in the skin into vitamin D). "Receiving enough sun exposure to make vitamin D isn’t always easy," says Michael F. Holick, Ph.D., M.D., professor of medicine at the Boston University Medical School. "It depends on the season, time of day, latitude of where you live, and your skin pigment. Darker skin absorbs sunlight less easily than lighter skin does. Therefore, teens of color likely require more sunlight exposure to create enough D." Cities in the north receive less direct sun than those in the south, particularly in the winter, which is why vitamin D deficiency is more likely among teens who live in cities at higher latitudes. And sunscreen, too, is a factor. "UVB rays are necessary for the body to activate the vitamin D creation process in the skin. Sunscreen blocks that out," Holick notes. Which isn’t to say you should sunbathe in order to get vitamin D. "Always use SPF and reapply during prolonged exposure," Callahan suggests.

So how can girls guard against skin cancer and still get enough vitamin D? First, check labels to see if any of your favorite foods are already fortified with the nutrient. Many products--like orange juice, cereal, and yogurt--have vitamin D added to them.

If your levels from food are insufficient, then "take a multivitamin that includes D," Callahan advises. "I tell my patients to get between 800 and 1,000 IUs [international units] per day which, between supplements and food, is easily achieved. There’s so much that modern medicine can’t fix, but solving vitamin D deficiency is simple."